Distributed sensing in an optical fiber network

ABSTRACT

A distributed optical fiber sensing system is provided. The system can include an interrogator, a transmission optical fiber, a sensing optical fiber, and a reflector. Each of the transmission optical fiber and the sensing optical fiber includes at least one circulator. The reflector is optically coupled to the transmission optical fiber for sensing separate regions along the sensing optical fiber.

TECHNICAL FIELD

The present disclosure relates generally to distributed sensing multiple regions in an optical fiber network.

BACKGROUND

Distributed sensing can involve sensing parameters at different locations along a sensing network. Each location may need to be sensed independently using signals that travel along the entire sensing network. Signals traveling along the entire sensing network can involve delays and a low data acquisition rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a distributed optical fiber sensing system according to one aspect.

FIG. 2 is a schematic view of the distributed optical fiber sensing system of FIG. 1 in which a region is depicted as being sensed according to one aspect.

FIG. 3 is a schematic view of a distributed optical fiber sensing system according to another aspect that includes four-port circulators.

FIG. 4 is a schematic view of a wellbore system that includes a distributed optical fiber sensing system according to one aspect.

FIG. 5 is a block diagram of an optical transmitter and an optical receiver according to one aspect.

FIG. 6 is a block diagram of an optical transmitter and an optical receiver according to another aspect.

FIG. 7 is a block diagram of an optical transmitter and an optical receiver according to another aspect.

DETAILED DESCRIPTION

Certain aspects and features relate to providing high bandwidth distributed sensing in an optical fiber network using optical backscatter and time-division multiplexing. The optical fiber network includes multiple regions, each region having a shorter length than the total length of an optical fiber. A higher pulse repetition rate of an optical signal can be used for sensing among multiple locations within the optical fiber network. Several parameters can be sensed in a distributed manner. Examples of parameters include temperature, acoustic, and strain. Distributed sensing can be performed selectively over a particular region of a sensing fiber with higher bandwidth than normally allowed with time-domain multiplexing interrogation methods.

In one aspect, a system includes at least two optical fibers. One optical fiber carries optical signals transmitted by an interrogator and another optical fiber is a sensing fiber that includes sensing regions in which optical backscatter is generated. Included in the optical fiber that carries the transmitted optical signals are one or more circulators and one or more reflectors. Each reflector is associated with a distinct optical wavelength at which the reflector reflects optical signals, and allows other wavelengths to pass through. An optical signal having an optical wavelength associated with a reflector can be reflected toward a circulator that can route the reflected optical signal to the sensing fiber in which the reflected signal creates a backscatter signal within a sensing region. The backscatter signal can be received by the interrogator and measured to determine the desired parameter. To interrogate a different region, the optical signal is configured to have a different optical wavelength, which is reflected by a different reflector and is routed to the different region.

Using certain aspects can result in the pulse rate of the interrogator being higher based on the length of the sensing region. For example, if the entire length of the sensing fiber is 5 kilometers (km), the maximum pulse rate of a interrogator in a non-region system may be limited to 20 kilohertz (kHz) to prevent interference from backscatter of different pulses. The pulse rate can be increased to 100 kHz by separating the sensing fiber into multiple regions that are each, for example, 1 km long. The optical fiber that carries the optical signal transmitted by the interrogator can be optimized for minimum attenuation and have a high non-linearity threshold to stimulated scattering to allow higher optical power to be transmitted. The sensing fiber can be optimized to generate more optical backscatter to increase the signal-to-noise ratio of the backscatter signal received by the interrogator. In some aspects, the sensing fiber can have a higher backscatter coefficient than the optical fiber that carriers the optical signal transmitted by the interrogator.

The interrogator can include a tunable light source. A sensing region can be selected in a round-robin fashion (e.g., ten seconds of interrogation for each region) or based on the needs of a particular implementation. An example of a tunable light source is the Colorado Source by Redfern Integrated Optics, which can be tunable across the C-band while providing a high coherence optical output.

These illustrative aspects and examples are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts. The following sections describe various additional features and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative aspects but, like the illustrative aspects, should not be used to limit the present disclosure.

FIG. 1 schematically depicts a distributed optical fiber sensing system according to one example. The system includes an interrogator 100 and two optical fibers: a transmission fiber 102 and a sensing fiber 104. The sensing fiber 104 is split into multiple regions S1-S5. Included in the sensing fiber 104 are circulators 106 a-e. In some aspects, the circulators 106 a-e can define the regions S1-S5. Included in the transmission fiber 102 are circulators 108 a-d and reflectors 110 a-d. The circulators 106 a-e and 108 a-d can be three-port circulators, but other types of circulators can be used. Each of the circulators 108 a-d can be communicatively coupled to a circulator of circulators 106 a-e by an optical fiber. Each reflector is associated with an optical wavelength, and can reflect optical signals at the optical wavelength and allow optical signals at other optical wavelengths to pass. Examples of a reflector include a fiber Bragg grating and a thin film reflector. Each of the transmission fiber 102 and the sensing fiber 104, along with the optical fibers coupling the circulators, can be an optical waveguide.

A three-port circulator is a device with three optical ports. An optical signal that enters a first port can exit from a second port. An optical signal that enters the second port can exit from a third port. An optical signal that enters the third port does not exit from any port and may be absorbed by the device.

The interrogator 100 includes an optical transmitter 112 and an optical receiver 114. The optical transmitter 112 can transmit an optical signal having a selected optical wavelength on the transmission fiber 102. The optical signal can pass through reflectors not associated with the selected optical wavelength and can be reflected by a reflector associated with the selected wavelength. The circulator preceding the reflector associated with the selected wavelength can route the reflected signal to a circulator in the sensing fiber 104. The sensing fiber circulator can route the reflected signal away from the optical receiver 114, which can create a backscatter signal toward the optical receiver 114 in one of the regions S1-S5. The sensing fiber circulator can allow the backscatter signal to pass toward the optical receiver 114 along the sensing fiber 104. The optical receiver 114 can receive the backscatter signal. The backscatter signal can be analyzed to measure a parameter in the region. For example, sensing region S2 can be selected by tuning the wavelength of the transmitter 112 to a value that is reflected by reflector 110 b. The reflector 110 b can be a fiber Bragg grating or any other type of component that can have a wavelength-selective reflection spectrum. For example, it can be based on dielectric film technology. Furthermore, even though the reflector 110 b is shown to be outside of the circulator, the reflector can be included within the same package as the circulator, which can save space.

Although five sensing regions are shown in FIG. 1, any number of sensing regions can be used. A system implemented with two sensing regions, for example, may include three circulators—two in the sensing fiber 104 and one in the transmission fiber 102—and one reflector in the transmission fiber 102. A particular sensing region can be selected by tuning the wavelength of the optical signal outputted by the optical transmitter 112 to be the reflected optical wavelength of a reflector associated with the particular sensing region.

A reflective wavelength of a particular reflector may not be known or the reflective wavelength of a particular reflector may change over time. Distributed optical fiber sensing systems according to some aspects can be calibrated to account for unknown or variable reflector characteristics. For example, the interrogator 100 can include a laser that sweeps across an entire wavelength range and the optical transmitter 112 outputs the optical signals onto the transmission fiber 102. The optical receiver 114 can receive signals from the sensing fiber 104 and record reflectivity peaks to identify the optical wavelength of peak reflectivity of each of the reflectors 110 a-d. In another example, broadband gratings (i.e., chirped gratings) can be used that have a wide optical frequency bandwidth near 100% reflectivity. In another example, the reflectors 110 a-d are athermal gratings that are not temperature sensitive and are less susceptible to changes. Athermal gratings may be devices packaged such that the usual temperature response of the reflector is compensated by a change in tension in the fiber using passive processes of systems based on the coefficient of thermal expansion and dimensions of the components inside the package.

FIG. 2 schematically depicts an example of the distributed optical fiber sensing system of FIG. 1 in which region S2 is sensed. Reflector 110 b is associated with an optical wavelength unique among the reflectors 110 a-d and that is associated within the interrogator 100 to sensing region S2. The optical transmitter 112 can be tuned to the optical wavelength corresponding to the optical wavelength associated with reflector 110 b and output an optical signal to the transmission fiber 102. The optical signal can travel along the transmission fiber 102 to the circulator 108 a, as indicated by line 116. The circulator 108 a receives the optical signal in a first port and routes the optical signal to exit the circulator 108 a at a second port. The reflector 110 a can be associated with a different optical wavelength such that the optical signal is not reflected by the reflector 110 a and travels along the transmission fiber 102 to a first port of the circulator 108 b. The circulator 108 b can route the optical signal received at the first port to exit a second port of the circulator 108 b. The optical signal can travel to the reflector 110 b that can reflect the optical signal to create a reflected optical signal that travels toward the second port of the circulator 108 b preceding the reflector 110 b, as shown by line 118.

The circulator 108 b can receive the reflected optical signal in the second port and route the reflected optical signal such that the reflected optical signal exits a third port of the circulator 108 b onto an optical fiber toward the circulator 106 b in the sensing fiber 104, as shown by line 120. The circulator 106 b can receive the reflected optical signal in a first port and can route the reflected optical signal out a second port onto the sensing fiber 104 toward the circulator 106 c and away from the optical receiver 114, as shown by line 124, through region S2. The reflected optical signal can generate a backscatter signal in region S2, as shown by line 126, and can be blocked by a third port of the next circulator 106 c. The backscatter signal can travel along the sensing fiber 104 and be received by the second port of the circulator 106 b that can route the backscatter signal out a third port of the circulator 106 b onto the sensing fiber 104. The backscatter signal can pass through the circulator 106 a in a similar manner toward the optical receiver 114. The optical receiver 114 can receive and process the backscatter signal as a sensing signal. The backscatter signal may be in the form of Rayleigh, Brillouin, or Raman. The backscatter signal may be coherent or incoherent light.

FIG. 3 schematically depicts an example of a distributed optical fiber sensing system according to another aspect. The system includes an interrogator 202, a transmission fiber 204, and a sensing fiber 206. The sensing fiber is divided into sensing regions S1-S3. The transmission fiber 204 includes reflectors 208 a-c. Each of the reflectors 208 a-c can be associated with an optical wavelength of signals that the reflector reflects such that parameters in each of the regions S1-S3 can be selectively and independently measured.

The transmission fiber 204 and the sensing fiber 206 are coupled to circulators 210 a-c. The circulators 210 a-c are four-port circulators. A four-port circulator is a device with four optical ports. An optical signal that enters a first port can exit from a second port. An optical signal that enters the second port can exit from a third port. An optical signal that enters the third port can exit from a fourth port. An optical signal that enters the fourth port does not exit from any port and can be absorbed by the device.

An optical signal outputted by the interrogator 202 onto the transmission fiber 204 can enter a first port of the circulator 210 a, which can cause the optical signal to exit a second port of the circulator 210 a toward the reflector 208 a. If the optical signal has an optical wavelength that the reflector 208 a reflects, the reflected optical signal can be received by the second port of the circulator 210 a, which can cause the reflected optical signal to exit the third port of the circulator 210 a into the region S1 and toward a fourth port of the circulator 210 a. Signals can be blocked by the fourth port of the circulators 210 a-c. A backscatter signal entering the third port of the circulator 210 a can be routed to exit a fourth port of the circulator 210 a toward the interrogator 202.

Distributed optical fiber sensing systems according to various embodiments can be implemented in a variety of applications and environments. FIG. 4 schematically depicts an example of a wellbore system 310 that includes a distributed optical fiber sensing system according to one aspect. The system 310 includes a wellbore 312 that penetrates a subterranean formation 314 for the purpose of recovering hydrocarbons, storing hydrocarbons, disposing of carbon dioxide, or pumping fluid into the well for stimulation (e.g., fracturing, acidizing, etc.) of producing zones or for storage or disposal. The wellbore 312 may be drilled into the subterranean formation 314 using any suitable drilling technique. While shown as extending vertically from the surface 316 in FIG. 4, in other examples the wellbore 312 may be deviated, horizontal, or curved over at least some portions of the wellbore 312. The wellbore 312 may be cased, open hole, contain tubing, and may include a hole in the ground having a variety of shapes or geometries. The wellbore system 310 includes a casing 318 extending through the wellbore 312 in the subterranean formation 314. Tubulars, such as production tubing, completion strings, and the like, are not shown but may also extend from the surface 316 in an inner area defined by the casing 318.

Some items that may be included in the wellbore system 310 have been omitted for simplification. For example, the wellbore system 310 may include a servicing rig, such as a drilling rig, a completion rig, a workover rig or other mast structure, or a combination of these. In some aspects, the servicing rig may include a derrick with a rig floor. Piers extending downwards to a seabed in some offshore implementations may support the servicing rig. Alternatively, the servicing rig may be supported by columns sitting on hulls or pontoons (or both) that are ballasted below the water surface, which may be referred to as a semi-submersible platform, rig, or drillship. In an off-shore location, a casing or riser may extend from the servicing rig to the sea floor to exclude sea water and contain drilling fluid returns. Other mechanical mechanisms that are not shown may control the run-in and withdrawal of a workstring in the wellbore 312. Examples of these other mechanical mechanisms include a draw works coupled to a hoisting apparatus, a slickline unit or a wireline unit including a winching apparatus, another servicing vehicle, and a coiled tubing unit.

The wellbore system 310 includes the distributed optical fiber sensing system that can sense parameters in multiple regions of the wellbore 312. The distributed optical fiber sensing system includes an interrogator 320 that has an optical transmitter 322 and an optical receiver 324. The distributed optical fiber sensing system also includes optical fibers that are a transmission fiber 326 and a sensing fiber 328. The sensing fiber 328 includes circulators 330 a-d, which can define regions in which parameters can be sensed. For example, a first region can be between circulator 330 a and circulator 330 b, a second region can be between circulator 330 b and circulator 330 c, a third region can be between circulator 330 c and circulator 330 d, and a fourth region can be preceded by circulator 330 d. The transmission fiber 326 includes circulators 332 a-c and reflectors 334 a-c, each associated with a different optical wavelength. Each of the circulators 332 a-c can be optically coupled to one of the circulators 330 a-d of the sensing fiber 328 by an optical fiber.

Each region can be independently sensed by the optical transmitter 322 outputting an optical signal having an optical wavelength corresponding to the reflector associated with the desired region. The optical receiver 324 can receive and analyze a backscatter signal generated in the desired region according to signal flow processes discussed above in connection with FIGS. 1-3.

As an example, a hydraulic fracturing process may be implemented in the wellbore system 310. The region or regions in which the fracturing is occurring can be selected for interrogation and the region or regions in which the fracturing is not occurring can be omitted because, for example, parameters in these latter regions may not be of interest. As fracturing proceeds to other regions, the desired regions can change and the interrogator 320 can modify the output signal for sensing parameters within the new regions of interest.

In other aspects, all regions can be sensed at the same time at a high bandwidth. Multiple optical sources, each tuned to the optical wavelength corresponding to a different sensing region can be used to output multiple optical signals. An interrogator can include a wavelength division multiplexing device that can separate backscattered light from each region and route the backscatter signal to separate detectors for processing into sensor parameters, such as temperature and vibration.

Various types of interrogators can be used in these and other aspects. FIG. 5 depicts by block diagram an example of an interrogator according to one aspect. The interrogator includes an optical transmitter 512 and an optical receiver 514. The optical transmitter 512 includes a tunable laser 516 coupled to the transmission fiber 102 for outputting optical signals as pulses to the transmission fiber 102. The optical transmitter 512 also includes a controller 518. In other aspects, the controller 518 may be in a central control unit that is separate from the optical transmitter 512. The optical receiver 514 includes electronics 520 and an optical detector 522 to detect distributed sensing signals from the sensing fiber 104.

The controller 518 can be communicatively coupled to the tunable laser 516 and to the electronics 520 in the optical receiver 514 for coordinating the timing of the pulses and other information with the tunable laser 516 and to the detector 522 in the optical receiver 514 through the electronics 520.

FIG. 6 depicts by block diagram an example of an interrogator according to another aspect. The interrogator includes an optical transmitter 612 and an optical receiver 614. The optical transmitter 612 includes a broadband source 616 and a controller 618, the latter of which can be in a central control unit in other aspects, rather than the optical transmitter 612. The broadband source 616 can be pulsed, as controlled by the controller 618, for outputting a broadband pulsed optical signal to the transmission fiber 102 that can be broken down into narrow wavelength ranges by reflectors. The optical receiver 614 includes electronics 620 communicatively coupled to the controller 618 for coordinating timing and other information. The optical receiver 614 also includes filters 622 and detectors 624. The filters 622 can be coupled to the sensing fiber 104 for separating out wavelengths of signals from various sensed sections of the sensing fiber 104. The detectors 624 can detect signals separately such that multiple sections of the sensing fiber 104 can be interrogated simultaneously.

FIG. 7 depicts by block diagram an example of an interrogator according to another aspect. The interrogator includes an optical transmitter 712 and an optical receiver 714. The optical transmitter 712 includes multiple tunable lasers 716 coupled to a combiner 718 that can combine multiple optical signals from the tunable lasers 716 and outputting the combined optical signal to the transmission fiber 102. The combined optical signal can include multiple optical signals at different wavelengths. The optical transmitter 712 also includes a controller 720 for controlling the tunable lasers 716 and coordinating timing and other information with the optical receiver 714. In other aspects, the controller 720 is in a central control unit rather than the optical transmitter 712. The optical receiver 714 includes electronics 722 coupled to the controller 720. The optical receiver 712 also includes filters 724 and receivers 726. The filters 724 can be coupled to the sensing fiber 104 for separating out wavelengths of signals from various sensed sections of the sensing fiber 104. The receivers 726 can detect signals separately such that multiple sections of the sensing fiber 104 can be interrogated simultaneously.

The foregoing description of certain aspects, including illustrated aspects, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of the disclosure. 

1. (canceled)
 2. The system of claim 7, wherein the sensing optical fiber is divided into the separate regions, each region having a smaller length than a total length of the sensing optical fiber.
 3. The system of claim 7, wherein the at least one circulator is a three-port circulator.
 4. The system of claim 3, further comprising a linking optical fiber between a first circulator of the transmission optical fiber and a second circulator of the sensing optical fiber.
 5. The system of claim 7, wherein the at least one circulator is a four-port circulator and is coupled to both the transmission optical fiber and the sensing optical fiber.
 6. (canceled)
 7. A system comprising: an interrogator that includes an optical transmitter and an optical receiver; a transmission optical fiber and a sensing optical fiber, each of the transmission optical fiber and the sensing optical fiber including at least one circulator; and a reflector optically coupled to the transmission optical fiber with which separate regions along the sensing optical fiber are sensed, wherein the reflector includes a plurality of reflectors, each reflector of the plurality of reflectors being associated with a separate region and with a distinctive optical wavelength, wherein the reflector associated with a first wavelength is responsive to an optical signal in the first wavelength from the optical transmitter by reflecting a reflected optical signal that includes the optical signal to a first circulator preceding the reflector along the transmission optical fiber and is transparent to optical signals of wavelengths other than the first wavelength by allowing the optical signals to pass through the reflector, wherein the first circulator is responsive to the reflected optical signal by routing the reflected optical signal to a second circulator of the sensing optical fiber for creating a backscatter optical signal with a selected region to be sensed and that is received by the optical receiver and measured.
 8. The system of claim 7, wherein the second circulator is operable for routing the reflected optical signal away from the optical receiver toward a sensing region and allowing the backscatter optical signal to pass toward the optical receiver.
 9. The system of claim 7, wherein the at least one circulator and the reflector are disposed in a wellbore.
 10. The system of claim 7, wherein the sensing fiber has a higher backscattering coefficient than the transmission fiber.
 11. The system of claim 7, wherein the optical transmitter is operable for outputting a signal that includes multiple wavelengths and the optical receiver is operable for receiving backscatter signals at different wavelengths for simultaneously sensing multiple regions of the sensing fiber.
 12. The system of claim 7, wherein the optical transmitter is operable for outputting a signal having a broadband spectrum and the optical receiver is operable for receiving backscatter signals at different wavelengths for simultaneously sensing multiple regions of the sensing fiber.
 13. A method, comprising: transmitting an optical signal having a selected optical wavelength associated with a desired region onto a transmission optical fiber; reflecting the optical signal by a reflector of the transmission optical fiber and that is associated with the selected optical wavelength to generate a reflected optical signal; routing, by a circulator that immediately precedes the reflector, the reflected optical signal to a sensing optical fiber divided into multiple regions that includes the desired region; routing the reflected optical signal into the desired region to generate a backscatter signal; and receiving the backscatter signal from the sensing optical fiber for measuring a parameter of the desired region.
 14. The method of claim 13, further comprising: allowing the optical signal to pass through at least one reflector o associated with a different optical wavelength than the selected optical wavelength.
 15. The method of claim 13, wherein routing the reflected optical signal into the desired region to generate the backscatter signal includes routing the reflected optical signal by a three-port circulator of the sensing optical fiber.
 16. The method of claim 13, wherein routing the reflected optical signal into the desired region to generate the backscatter signal includes routing the reflected optical signal by the circulator that immediately precedes the reflector, the circulator being a four-port circulator that is coupled to the transmission optical fiber and the sensing optical fiber.
 17. The method of claim 13, wherein the transmission fiber includes a plurality of reflectors, wherein each reflector is associated with a separate region and with a distinctive optical wavelength.
 18. A system comprising: a transmission optical fiber; a sensing optical fiber divided into separate regions; a reflector associated with an optical wavelength and coupled to the transmission optical fiber; and a circulator immediately preceding the reflector for routing a reflected optical signal from the reflector to a selected region of the separate regions in the sensing optical fiber with which the selected region can be sensed.
 19. The system of claim 18, wherein at least part of the transmission optical fiber and at least part of the sensing optical fiber are located in a wellbore through a subterranean formation.
 20. The system of claim 19, further comprising an interrogator with an optical transmitter coupled to the transmission optical fiber and an optical receiver coupled to the sensing optical fiber.
 21. The system of claim 20, wherein the optical receiver is operable for receiving a backscatter signal generated in the selected region by the reflected optical signal traveling away from the optical receiver. 